1. Field of the Invention
The present invention relates to an objective lens element used for performing at least one of recording, reproducing, and erasing of information on an optical information medium such as an optical disc, and an optical head device including the objective lens element.
2. Description of the Background Art
In recent years, research and development has been actively carried out concerning high-density optical discs that have an increased recording density and thus have an increased storage capacity. A standard of such a high-density optical disc is Blu-Ray (R) Disc (hereinafter, referred to as “BD”) in which the image side numerical aperture (NA) of an objective lens element is set to about 0.85 and the thickness of a protective base plate formed on an information recording surface of an optical disc is set to about 0.1 mm.
In addition to the above BD standard, a standard (so-called DVD standard) in which a red laser beam with a wavelength of about 680 nm is used and the thickness of a protective base plate formed on an information recording surface of an optical disc is set to about 0.6 mm, and a standard (so-called CD standard) in which an infrared laser beam with a wavelength of about 780 nm is used and the thickness of a protective base plate is set to about 1.2 mm, have also been used. Various objective lens elements that are compatible with at least two of these three standards have been developed.
For example, an objective lens element that is compatible with two types of information storage media, BD and DVD, and an objective lens element that is compatible with three types of information storage media, BD, DVD, and CD, are known (for the latter, for example, see Japanese Laid-Open Patent Publication No. 2010-170694).
In the objective lens element that is compatible with two types of information storage media, BD and DVD, a first surface is divided into two regions which are concentric about a symmetry axis (optical axis), a compatible region which performs aberration compensation for two types of wavelengths for BD and DVD is formed in the region close to the optical axis, and an outer region optimized for BD is formed outside the compatible region. The compatible region has a diffraction structure and achieves spherical aberration compensation for two types of formats of BD and DVD by using a difference in diffraction angle caused by a difference in wavelength. It should be noted that the first surface refers to a surface located closer to a light source, among two optically functional surfaces of the objective lens element. In other words, the first surface is an incident surface of the objective lens element. In addition, a surface opposed to the first surface is referred to as a second surface. In other words, the second surface is an exit surface of the objective lens element.
In the objective lens element that is compatible with three types of information storage media, BD, DVD, and CD, a first surface is divided into three regions which are concentric about the optical axis, a compatible region which performs aberration compensation for three types of wavelengths for BD, DVD, and CD is formed in the region closest to the optical axis, another compatible region which performs aberration compensation for two types of wavelengths for BD and DVD is formed outside the compatible region, and an outer region optimized for BD is formed outside the other compatible region. The compatible region has a diffraction structure and achieves spherical aberration compensation for three types of formats of BD, DVD, and CD by using a difference in diffraction angle caused by a difference in wavelength.
However, when the objective lens element having the above configuration is used, it is necessary to increase the diffraction power of the compatible region in order to ensure a sufficient working distance (in particular, when DVD or CD is used) while aberration compensation is performed for a plurality of formats.
When the diffraction power is increased, the interval of the periodic diffraction structure in the compatible region narrows as distance from the optical axis of the objective lens element increases.
When the interval of the diffraction structure narrows, the diffraction efficiency decreases. As a result, an amount of light passing through the compatible region decreases. Thus, in the conventional objective lens element, performance sufficient to perform recording, reproducing, and erasing of information on an optical information medium such as an optical disc is not obtained.
Further, the interval of the diffraction structure narrows as distance from the optical axis increases. When the interval of the diffraction shape narrows, an amount of light at a peripheral portion of the compatible region greatly decreases. When DVD or CD is used, it is necessary to form a predetermined convergence spot only with light passing through the compatible region. Thus, when an amount of light which determines a numerical aperture for DVD or CD decreases, the numerical aperture effectively decreases. As a result, reproducing/recording performance deteriorates, since a convergence spot on the optical disc effectively enlarges.